1. Field of the Invention
The present invention relates generally to tunneling magnetoresistance elements, and more particularly to a tunneling magnetoresistance element having high sensitivity, and to a magnetic sensor, magnetic head, and magnetic memory using the tunneling magnetoresistance element.
2. Description of the Related Art
A magnetic sensor having a tunneling magnetoresistance (TMR) effect element is disclosed in Physics Letters, Vol. 54A, No. 3, 225 (1975). A TMR element exhibits a better magnetoresistance effect than that exhibited by other magnetoresistance (MR) elements currently known. Thus, the TMR element is of interest for study in the development of reproducing magnetic heads.
Referring to FIG. 1, the TMR element has a structure into which a dielectric insulating layer 310 is sandwiched between a magnetic layer 110 and a magnetic layer 210. When the two magnetic layers 110, 210 have different coercive forces, the respective directions of magnetization of the individual magnetic layers change between a parallel and an antiparallel relation in dependence on the change in the external magnetic field 800. Additionally, when a bias voltage V is applied between these two magnetic layers 110, 210, a tunnel current flows through the insulating layer 310, and a tunnel resistance R for the device can be defined by R=V/I. The tunnel resistance R changes in dependence upon whether the directions of magnetization of the magnetic layers 110, 210 are parallel or antiparallel. A TMR element that exhibits the change in tunnel resistance R with changing external magnetic field 800 can be used as a magnetic sensor.
In conjunction with the MR element of the prior art, a circuit construction has been used in which an electric current bias is effected between the terminals of the element to sense the voltage change across the element caused by the change in the external magnetic field. However, when the conventional MR element is replaced by a TMR element, there is a large difference between the impedances of the terminals. For example, as described in J. Appl. Phys. Vol. 79, No. 8, 4724 (1996), a TMR element has a terminal impedance of several kiloohms, while an MR element has a terminal impedance of about tens of ohms. The main reason for the difference resides in the controllability of the method of forming a tunnel barrier layer.
As described in the above J. Appl. Phys. article, a tunnel barrier of about 1 to 2 nanometers, as necessary, is created by oxidizing a metal film of Al or the like having a similar thickness. A sufficient average thickness is required to form a tunnel barrier layer which has little leakage current due to pin holes. As a result, the tunnel resistance cannot be lowered below a certain value.
Moreover, variation in the tunnel resistance of the elements which are thus fabricated is large, because the tunnel resistance depends exponentially on the thickness of the tunnel barrier layer, whereby a small variation in the dielectric layer thickness appears as a large variation in the tunnel resistance. This tunnel resistance variation is especially noted among plural elements formed over different substrates, in comparison with the variation of tunnel resistance of plural elements formed over a common substrate, because the reproducibility and controllability of the oxidation process is insufficient.